Effective handling of domestic sewage and industrial wastewater is an extremely important aspect of increasing the quality of life and conservation of clean water. The problems associated with simply discharging wastewater in water sources such as rivers, lakes and oceans, the standard practice up until about a half century ago, are apparent—the biological and chemical wastes create hazards to all life forms including the spread of infectious diseases and exposure to carcinogenic chemicals. Therefore, wastewater treatment processes have evolved into systems ranging from the ubiquitous municipal wastewater treatment facilities, where sanitary wastewater from domestic populations is cleaned, to specialized industrial wastewater treatment processes, where specific pollutants of various wastewater applications are addressed.
In general, wastewater treatment facilities use multiple treatment stages to clean water so that it may be safely released into bodies of water such as lakes, rivers, and streams. Presently, many sanitary sewage treatment plants include a primary treatment phase where mechanical means are used to remove large objects (e.g., bar screens), and a sand or grit channel where sand, grit and stones settle. Some treatment systems also include a stage where certain fats, greases and oils may float to the surface for skimming. The wastewater is then sent to a secondary biological activated sludge treatment phase. Activated sludge processes involve aerobic biological treatment in an aeration tank, typically followed by a clarifier/settling tank. The clarifier/settling tank effluent may or may not undergo further treatment steps before being discharged, and the sludge is recycled back to the aeration tank for further treatment, or is further treated before being disposed of in either a landfill, incinerator, or used as fertilizer if there are no toxic components.
In the aeration tank, air is added to the mixed liquor (a mixture of the feed wastewater and a large quantity of bacteria). The oxygen from the air is used by the bacteria to biologically oxidize the organic compounds that are either dissolved or carried in suspension within the wastewater feed. Biological oxidation is typically the lowest cost oxidation method available to remove organic pollutants from wastewater and is the most widely used treatment system for wastewater contaminated with biologically treatable organic compounds. Wastewaters that contain biologically refractory, or hard-to-treat, organic compounds or wastewaters that contain inorganic constituents are typically not able to be treated adequately by a conventional biological wastewater treatment system and often require more expensive methods to remove the pollutants.
The mixed liquor effluent from the aeration tank typically enters a clarifier/settling tank where waste sludge (concentrated mixed liquor suspended solids) settles by gravity. However, based on the wastewater and economic needs, some biological oxidation systems use a different treatment method to remove the solids from the wastewater effluent. The clarifier/settling tank can be replaced with a membrane (membrane biological reactor), or another unit operation such as a dissolved air flotation device can be used. The liquid effluent from the clarifier/settling tank, membrane or dissolved air flotation device is either discharged or given further treatment prior to discharge. The solids that are removed from the mixed liquor are returned to the aeration tank as return activated sludge for further treatment and in order to retain the bacteria in the system. Some portion of this return activated sludge is periodically removed from this recycle line in order to control the concentration of bacteria in the mixed liquor.
Increasingly, sanitary wastewater is being treated using membrane biological reactor technology, which offers improved effluent quality, a smaller physical footprint (more wastewater can be treated per square foot of treatment area), increased tolerance to upsets, improved ability to process hard-to-treat wastewaters (for example, wastewaters containing high total dissolved solids cannot be treated in a conventional clarifier/settling tank and requires significantly more difficult-to-operate solids settling device such as a dissolved air flotation device or some other solids removal system) and a variety of other operational advantages. However, membrane biological reactors often present problems with membrane fouling and foaming that do not occur in conventional systems using clarifiers. Membrane fouling is typically caused by extra-cellular polymeric compounds that result from the break-down of the biological life forms in the mixed liquor suspended solids.
One recent advance in conventional industrial biological wastewater treatment plant technology includes the addition of powdered activated carbon particles to the mixed liquor. In these processes, certain organic and inorganic compounds are physically adsorbed to the surface of the powdered activated carbon particles. One example of a known powder activated carbon system is offered by Siemens Water Technologies under the trademark “PACT®.” Powdered activated carbon has been used in conventional biological treatment plants because of its ability to adsorb biologically refractory organic and inorganic compounds, thereby providing an effluent with lower concentrations of these pollutants. Inclusion of powdered activated carbon in the mixed liquor provides a number of operational benefits. The carbon provides the advantages of a suspended media biological treatment system which include increased pollutant removal and increased tolerance to upset conditions. Additionally, the carbon allows the biologically refractory organic materials to adsorb onto the surface of the carbon and to there be exposed to the biology for a significantly longer period of time than in a conventional biological treatment system, thereby providing benefits similar to that of a fixed film system. The carbon also allows for the evolution of specific strains of bacteria that are more capable of digesting the biologically refractory organic materials. The fact that the carbon is continuously recycled back to the aeration tank with the return activated sludge means that the bacteria can continually work on digesting the biologically refractory organic compounds adsorbed onto the surface of the carbon. This process also results in biological regeneration of the carbon and allows the carbon to remove significantly more biologically refractory compounds than it could in a simple packed bed carbon filter system which would also require frequent replacement or costly physical regeneration of the carbon once the adsorption capacity of the carbon is exhausted. The carbon in the mixed liquor can also adsorb and remove from the effluent the inorganic compounds that are not treatable by biological oxidation. However, to date, membrane biological reactors have not been utilized commercially with powdered activated carbon addition. There has been some use of powdered activated carbon in surface water treatment systems that utilize membranes for filtration. However, these surface water treatment systems using membranes and powdered activated carbon have been reported to have problems with the carbon abrading the membranes and the carbon permanently plugging and/or fouling the membranes.
Industrial wastewater that must be treated prior to discharge or reuse often include oily wastewaters, which can contain emulsified hydrocarbons. Oily wastewaters can come from a variety of industries including steel and aluminum industries, chemical processing industries, automotive industries, laundry industries, and crude oil recovery and refining industries. As discussed above, a certain amount of non-emulsified oils and other hydrocarbons may be removed in primary treatment processes, where floating oils are skimmed from the top. However, biological secondary wastewater processes are generally employed to remove the remaining oils from wastewater. Typical hydrocarbons remaining after primary treatment can include lubricants, cutting fluids, tars, grease, crude oils, diesel oils, gasoline, kerosene, jet fuel, and the like. The remaining hydrocarbons in the wastewater can be present in the range of from tens to thousands of parts per million. These hydrocarbons must be removed prior to discharge of the water into the environment or reuse of the water in the industrial process. In addition to governmental regulations and ecological concerns, efficient removal of the remaining hydrocarbons also has benefits, as adequately treated wastewater may be used in many industrial processes and eliminate raw water treatment costs and reduce regulatory discharge concerns.
Commercial deployment of membrane biological reactors in the treatment of oily/industrial wastewater has been very slow to develop, mainly due to maintenance problems associated with oil and chemical fouling of the membranes. Testing of industrial/oily wastewater treated in a membrane biological reactor having powdered activated carbon added to the mixed liquor indicated the same treatment advantages as observed in conventional biological wastewater treatment systems including powdered activated carbon. It was also noted that the advantages of using a membrane biological reactor can also achieved. However, the side-by-side comparison of membrane biological reactors with and without the addition of powdered activated carbon demonstrated that the membrane biological reactor with powdered activated carbon addition had all of the treatment advantages of the two systems whereas the membrane biological reactor without the carbon addition was very difficult if not impossible to operate because of residual oil and extra cellular polymeric compounds fouling the membranes. The testing further demonstrated that while the addition of powdered activated carbon provided a very viable biological wastewater treatment system, the carbon had the deleterious effect of a significant amount of abrasion to and non-reversible fouling of the membranes. This abrasion and non-reversible fouling was significant enough to result in this system being very costly to operate (because of the significantly decreased life expectancy of the membranes).
In certain types of water treatment processes not employing membrane biological reactors, granular activated carbon is used as an adsorbent medium in a fixed bed. For example, U.S. Pat. No. 5,126,050 to Irvine et al. describes a multistage process including a granular activated carbon filtration stage to adsorb organic contaminants from sources such as wastewater or spent granular activated carbon. The granular activated carbon is maintained in a tank, and the influent is provided at the bottom through a distributor to prevent fluidizing the granular activated carbon bed. Another example is Japanese Patent Application Number JP10323683 to Inoue et al. describes a water purification treatment method for obtaining potable water from raw water. The system includes a granular activated carbon bed within a water tank. Organic matter is adsorbed on granular activated carbon that includes aerobic bacteria adhered thereon. This system and process is described as treating water such as river water, lake water, pond water, and groundwater, rather than wastewater. In these systems the operating costs of having to either replace or regenerate the carbon once its adsorption capacity is exhausted is typically prohibitive. Furthermore, these systems do not utilize biological oxidation to either reduce the adsorptive capacity requirements or to regenerate the carbon.
In addition, European Patent Publication Number EP1258460 to Beyers et al. discloses a method for the biodegradation of oxygenates in groundwater or other water streams using packed beds of granular carbon inoculated with biodegraders. In particular, Beyers et al. discloses preferred embodiments where the granular carbon is inoculated with MTBE-degrading micro-organisms. While this system combines carbon adsorption of contaminants with biological oxidation, it was designed for a specific hard-to-treat wastewater need that allows for a significantly higher cost-of-treatment than is typical for biological treatment systems. Furthermore, it uses packed bed technology rather than a biological treatment system (such as membrane biological reactors).
The use of any of these above cited examples is limited to those applications where treatment cost is not an issue, as they are relatively expensive treatment methods. Additionally, none of them include the cost and operational advantages of a membrane biological reactor combined with carbon adsorption.
In other water purification applications, granular activated carbon is held in suspension in a body of water to be treated. For instance, European Patent Publication Number EP0543579 to Ford describes a process of removing pesticide residues and other organic substances from water. Activated carbon is added in a continuous manner, as spent activated carbon is continuously removed along with treated water. However, the system disclosed in the Ford reference requires a large amount of granular activated carbon to remove contaminants (pesticide residues). There is no biological mechanism for removing these contaminants.
Japanese patent documents JP62286591 and JP63016096 disclose organic waste water treatment methods involving a mixture of activated sludge slurry and granular activated carbon. These methods, disclosed prior to the development of immersed membrane biological reactors, use granular activated carbon in combination with ultra-filtration or reverse osmosis membranes in a downstream membrane system. These references address problems associated with organic wastewater (e.g., human waste) and do not practically address industrial wastes such as oily wastewater containing hydrocarbons. Additionally, this system was developed just as membrane biological reactors were first being tested, and its purpose was to prevent refractory organic compounds in the mixed liquor from fouling the membranes. It was speculated by the inventors that the use of membranes in place of a conventional clarifier/settling tank would ultimately result in toxic concentrations of organic compounds and interfere with the normal biological oxidation process in the aeration tanks. However, in practice it was found that this did not occur and as a result this technology was not commercialized. Furthermore, the primary application of the systems described in these references is treatment of sanitary wastewater having relatively low chemical oxygen demand (COD) compounds (e.g., COD to biological oxygen demand (BOD) ratios of about 2:1), in contrast to industrial wastewaters where the COD to BOD ratios are significantly higher.
As used herein, biologically refractory compounds refer to those types of COD compounds (organic and/or inorganic) in wastewater that typically do not break down with micro-organisms within twelve hours of residence time.
Further, as used herein, bio-inhibitory compounds refer to compounds (organic and/or inorganic) in wastewater that inhibit the biological breakdown process.
Therefore, it is an object of the present invention to provide a process and apparatus in a membrane biological reactor system that employs granulated activated carbon to adsorb biologically refractory organic compounds and inorganic compounds onto the surface of granulated activated carbon and to maintain the granulated activated carbon in the biological reactor system upstream of the membrane separation step. This will provide the advantages of a suspended media membrane biological reactor which has had powdered activated carbon added to the mixed liquor, without the abrasion and fouling that accompanies the carbon in the mixed liquor coming in contact with the membranes.
Another object of the invention is to provide such a process and system that is efficient, that requires a minimal capital investment for installation, and has low operating costs.
Yet another object of the invention is to implement such a process and system that is particularly well suited for treatment of industrial and oily wastewaters.